Optogenetics is the combination of genetic and optical methods used to control specific events in targeted cells of living tissue, even within freely moving mammals and other animals, with the temporal precision (millisecond-timescale) needed to keep pace with functioning intact biological systems. The hallmark of optogenetics is the introduction of fast light-responsive opsin channel or pump proteins to the plasma membranes of target neuronal cells that allow temporally precise manipulation of neuronal membrane potential while maintaining cell-type resolution through the use of specific targeting mechanisms. Among the microbial opsins which can be used to investigate the function of neural systems are the halorhodopsins (NpHRs), used to promote membrane hyperpolarization when illuminated, and the channelrhodopsins, used to depolarize membranes upon exposure to light. In just a few short years, the field of optogenetics has furthered the fundamental scientific understanding of how specific cell types contribute to the function of biological tissues, such as neural circuits, in vivo. Moreover, on the clinical side, optogenetics-driven research has led to insights into the neurological mechanisms underlying complex mammalian behaviors such as anxiety, memory, fear, and addiction.
In spite of these advances, use of optogenetic methods in animals suffers from the significant drawback of requiring the animal to either be tethered to a light source or to have a light source surgically implanted into the animal. Moreover, when optogenetic methods are used to alter the function of neurons in the brain, a light source must be placed in proximity to those neurons. This requires drilling a hole in the animal's skull and also presents practical difficulties when the brain region of interest is located deep within the brain itself. Since light poorly passes through neural tissue, this necessitates inserting a fiber optic light source into the brain, which can result in unintended damage to surrounding brain tissue.
What is needed, therefore, is a method to non-invasively deliver light to neurons located within the brain and the peripheral nervous system of animals expressing light-responsive opsin proteins on the plasma membranes of neural cells.
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